1. Field of the Invention
The present invention relates to an apparatus and method for lubricating a feed mechanism of a forming machine. Description of the
2. Related Art
Conventionally, in a forming machine, such as an injection molding machine, resin heated and melted in a heating cylinder is injected into a cavity of a mold apparatus under high pressure so that the cavity is filled with the molten resin. The molten resin is then cooled and solidified within the cavity so as to produce a molded article.
The mold apparatus consists of a stationary mold and a movable mold. A mold clamping apparatus for advancing and retracting the movable mold is provided so as to bring the movable mold into contact with the stationary mold and separate the same from the stationary mold, to thereby effect mold closing, mold clamping, and mold opening.
The mold clamping apparatus has a toggle mechanism for advancing and retracting the movable mold. The toggle mechanism is operated through drive of a drive source, such as an electric motor or a servomotor, disposed at a drive section.
FIG. 1 is a sectional view of a drive section of a conventional mold clamping apparatus.
In FIG. 1, reference numeral 51 denotes a servomotor serving as a drive source. The servomotor 51 is attached to an unillustrated stationary member, such as a toggle support, and has a rotary shaft 52. The front end (right-hand end in FIG. 1) of the rotary shaft 52 is coupled to the rear end (left-hand end in FIG. 1) of a ball-screw shaft 56 via a coupling 53. A key groove is formed on each of the outer circumferential surface of the rotary shaft 52, the outer circumferential surface of the ball-screw shaft 56, and the inner circumferential surface of the coupling 53; and keys 54 are fitted into the key grooves. Thus, rotation of the rotary shaft 52 is transmitted to the ball-screw shaft 56 via the coupling 53.
The ball-screw shaft 56 is rotatably supported by bearings 57 accommodated within a bearing housing 58, which is attached to the unillustrated stationary member, such as a toggle support. The outer rings of the bearings 57 are retained by means of a plate 59 attached to the bearing housing 58. The ball-screw shaft 56 is fixedly attached to the inner rings of the bearings 57 by means of a nut 60, so that the ball-screw shaft 56 cannot move along the axial direction.
A screw groove is formed on the outer circumference of the ball-screw shaft 56 over substantially the entire length thereof, and the ball-screw shaft 56 is in screw-engagement with a ball-screw nut 61. The ball-screw shaft 56 and the ball screw nut 61 constitute a ball-screw-type feed mechanism. The ball-screw nut 61 is attached to a cross head 62 of a toggle mechanism, which is slidable along guide bars 63.
Therefore, when the servomotor 51 is operated, rotation of the rotary shaft 52 is transmitted to the ball-screw shaft 56, and the ball-screw nut 61 in screw-engagement with the ball-screw shaft 56 moves along the axis of the ball-screw shaft 56. As a result, the cross head 62 is moved leftward and rightward in FIG. 1. When the cross head 62 is advanced (moved rightward in FIG. 1), the toggle mechanism extends so as to advance an unillustrated movable platen, to thereby perform mold closing and mold clamping. When the cross head 62 is retracted (moved leftward in FIG. 1), the toggle mechanism contracts so as to retract the movable platen, to thereby perform mold opening.
Since large torque is required to effect mold closing, mold clamping, and mold opening, heavy load acts on a ball screw that is constituted by the ball-screw shaft 56 and the ball-screw nut 61. In view of this, grease serving as a lubricant is supplied to the ball screw in order to enable smooth movement of the ball screw serving as a feed mechanism and prevent wear of the ball screw to thereby prolong the service life of the ball screw.
However, in the conventional ball screw serving as a feed mechanism, since grease is used for lubrication, maintaining a uniform film of lubricant at the contact surfaces between the balls and the screw is difficult. Consequently, lubrication conditions at respective portions of the ball screw become uneven. In particular, when the stroke of movement of the ball-screw shaft relative to the ball-screw nut is short, the grease is pushed out from the contact surfaces between the balls and the screw, and therefore, maintaining the lubricant film is difficult. As a result, there arises a variation in service life among the respective portions of the ball screw, thereby shortening the overall service life of the ball screw.
When the supply rate of grease is set greater than a required rate in order to guarantee that grease is distributed sufficiently to respective portions of the ball screw, consumption of grease increases. In general, grease is expensive, and therefore, the increased consumption of grease renders maintenance cost of the forming machine extremely high. Further, when the supply rate of grease is increased, excessive grease overflows, scatters, and contaminates the forming machine and an area surrounding the forming machine.
Further, when the ball screw is used for a long period of time, iron particles generated due to wear contaminate grease. If lubrication is performed by use of grease containing iron particles, contact surfaces are abraded by the iron particles. Therefore, grease containing iron particles must be discharged as soon as possible. However, when the supply rate of grease is increased in order to discharge grease containing iron particles as soon as possible, grease is discharged from the ball screw at a high rate, resulting in increased maintenance cost and contamination of the forming machine and an area surrounding the forming machine, as described above.
Moreover, since grasping the progress of wear of the ball screw is difficult, the service life of the ball screw cannot be predicted accurately. Therefore, in some cases the ball screw is used even after its service life has been reached. As a result, the feed mechanism of the forming machine operates erratically, whereby the accuracy of formed products decreases, and other components of the forming machine are affected adversely. Meanwhile, when the ball-screw shaft and the ball-screw nut are replaced prematurely in order to avoid use of the ball screw beyond its service life, the maintenance cost of the forming machine increases.
In view of the foregoing, there has been proposed a method for measuring the amount of iron contained in grease adhering to the ball-screw shaft and the ball-screw nut and predicting the service life of the ball screw. However, this method requires stopping a forming machine so as to collect grease adhering to the ball-screw shaft and the ball-screw nut. When the frequency of operation of collecting grease is increased in order to improve prediction accuracy, the total stoppage time of the forming machine increases, and the productivity of the forming machine decreases. Meanwhile, the frequency of operation of collecting grease is decreased in order to shorten the total stoppage time of the forming machine, prediction accuracy deteriorates.